1. Field of the Invention
The present invention relates to processes for the separation of acid from sugars obtained from the acid hydrolysis of cellulosic and/or hemicellulosic material. The present invention further relates to a method for removing the heavy metals associated with the acid/sugar solutions that are obtained from municipal solid waste (MSW) or sewage sludge which may contain cellulosic and/or hemicellulosic materials.
2. Related Art
U.S. Pat. No. 5,580,389 discloses methods for separating acids and sugars resulting from strong acid hydrolysis of biomass containing cellulose and hemicellulose. The cellulose and hemicellulose is first decrystallized and then hydrolyzed to produce a hydrolysate containing both sugars and acid. Silica present in the biomass can then be removed for further processing. The remaining solids are then subjected to a second decrystallization and hydrolyzation to optimize the sugar yields. A method for separating the sugars from the acid in the hydrolysate is also disclosed. The resultant sugar stream containing both hexoses and pentoses can then be fermented. In particular, the method of separating the sugars from the hydrolysate involves the use of a resin separation unit wherein the sugars are adsorbed on a strong acid resin. Liquids are then added to the resin and an acid stream containing less than 2% sugar is obtained.
U.S. Pat. Nos. 5,407,580 and 5,538,637 disclose a process for separating an ionic component such as acid from a nonionic component such as sugar in polar solutions using ion exclusion technology. In particular, the methods involve control of dispersion, caused by shrinkage of resin by acid within ion exchange columns. The continuous method employs an acid resin which is contacted with an acid-sugar solution whereby a first fraction comprises an enriched fraction of acid and later fractions comprise an enriched fraction of sugar.
U.S. Pat. Nos. 5,407,817 and 5,571,703 disclose methods of processing municipal solid waste to recover materials that may be recycled and to produce ethanol from the cellulosic component. One particular problem of municipal solid waste is the heavy metal component which may inhibit the fermentation of sugars by yeast or other fermentative microorganism. According to these patents, the heavy metals can be removed in any one of two different methods. In the xe2x80x9cfront endxe2x80x9d process, the heavy metals are removed by treating the shredded cellulosic component with dilute sulfuric acid at about 40 to 100xc2x0 C. to solubilize substantially the heavy metals. The insoluble component of the reaction mixture comprising the cellulose may then be recovered and processed to give ethanol. In the xe2x80x9cback endxe2x80x9d process, the shredded cellulosic component is directly hydrolyzed with concentrated sulfuric acid. After dilution with water and cooking at elevated temperatures, the insoluble lignin is removed. It was discovered that heavy metals were associated with the insoluble lignin which allows for the removal of the heavy metals from the sugar containing solution.
U.S. Pat. No. 5,651,895 discloses methods of removing heavy metals from certain waste waters by adding an iron salt, and then a base to increase the pH to a level of about 8 to 10. As the base is added, a precipitate containing the heavy metals is formed which is then removed by standard filtration techniques. The process can be carried out continuously or batch wise.
The invention relates to a method for separating acid and sugars obtained from liquids resulting from the acid hydrolysis of at least one of cellulosic and hemicellulosic materials, comprising adding the liquids to a separation unit comprising a bed of anionic exchange or exclusion chromatographic material whereby the acid is adsorbed onto the chromatographic material, thereby producing a series of first fractions comprising sugar solutions and a later series of fractions comprising acid solutions.
The present invention represents a significant departure compared to the prior art which employed a cationic resin to adsorb the sugar. In this prior art embodiment, the sugar was treated as the xe2x80x9cproductxe2x80x9d of the separation and was obtained in relative high purity compared to the acid which eluted first from the resin. The acid fractions contained impurities as little or no fractionation of the acid occurred. In contrast, according to the present invention, the acid is adsorbed onto a bed of anionic exchange or exclusion chromatographic material which results in the acid eluting after the sugar has eluted from the bed. Thus, according to the present invention, the acid is treated as the product of the separation and is obtained in higher purity and concentration compared to when a cationic resin is employed. Since acid is much more expensive to concentrate (for recycling in the process) compared to sugar (for fermentation), the use of an anionic exchange or exclusion chromatographic material achieves a significant cost savings in terms of energy requirements and capital costs compared to when a cationic resin is employed.
Since the sugar is not retarded on the anionic exchange or exclusion chromatographic material, it elutes first and is contaminated by some acid and heavy metals. The heavy metal contamination may become significant when the cellulosic and/or hemicellulosic material is derived from MSW. In this embodiment, the residual acid and heavy metals may be removed from the combined sugar fractions by treatment with lime which both neutralizes the residual acid and precipitates the heavy metals to give gypsum containing the heavy metals.
Thus, the invention is also related to a method for separating acid and sugars obtained from liquids resulting from the acid hydrolysis of cellulosic and/or hemicellulosic materials and removal of residual acid and heavy metals from the resultant sugar solution, comprising
(a) adding the liquids to a separation unit comprising a bed of anionic exchange or exclusion chromatographic material whereby the acid is adsorbed onto the chromatographic material, thereby producing a series of first fractions comprising sugar solutions and a later series of fractions comprising acid solutions; and
(b) treating the sugar solutions with lime to neutralize any residual acid and precipitate the heavy metals that may be present in the combined sugar solutions.
The invention is also related to a method for separating acid and sugars obtained from liquids resulting from the acid hydrolysis of cellulosic and/or hemicellulosic materials and removal of residual acid and heavy metals from the resultant sugar solution, comprising
(a) adding the liquids to a separation unit comprising a bed of anionic exchange or exclusion chromatographic material whereby the acid is adsorbed onto the chromatographic material, thereby producing a series of first fractions comprising sugar solutions and a later series of fractions comprising acid solutions;
(b) combining the first fractions comprising the sugar solutions; and
(c) treating the combined sugar solutions with lime to neutralize any residual acid and precipitate the heavy metals that may be present in the combined sugar solutions.
The present invention provides for the continuous removal of heavy metals. The heavy metals contaminate both the acid and sugar fractions. Thus, a fraction of the heavy metals are contained in the acid fractions which are combined, concentrated and recycled. With repeated recycling of the acid and removal of a fraction of the heavy metals in the combined sugar solutions, a steady state concentration of heavy metals in the acid fractions will be achieved.
Thus, the invention also relates to a continuous method for removal of heavy metals from a sugar and acid solution, comprising
(a) hydrolyzing a sample comprising heavy metals and cellulose and/or hemicellulose with concentrated acid to give a partially digested mixture;
(b) diluting the partially digested mixture with water (which may comprise raw sewage, septage, sewage sludge, waste water sludges, commercial waste streams and/or leachate from a land fill) and heating for a time sufficient to hydrolyze substantially the cellulose and/or hemicellulose to give a sugar/acid solution comprising heavy metals;
(c) adding the sugar/acid solution comprising heavy metals to a simulated moving bed separation unit comprising a bed of anionic exchange or exclusion chromatographic material whereby the acid is adsorbed onto the chromatographic material, thereby producing a series of first fractions comprising sugar solutions and a later series of fractions comprising acid;
(d) combining the first fractions comprising the sugar solutions;
(e) treating the combined sugar solutions with lime to neutralize any residual acid and precipitate the heavy metals that may be present in the combined sugar solutions; and
(f) combining and concentrating the fractions containing acid.
The advantages of the present invention are manifold:
(1) The acid concentration of the combined acid fractions is higher (about 8-16%, more preferably 10-16%) compared to when a cationic exchange resin is used (about 4.5-7%), which lowers the size, cost, energy consumption and overall operating costs of the acid evaporator which concentrates the acid and recycles it in the process. Besides an acid evaporator, the acid may also be concentrated by other methods such as electrodialysis and extraction.
(2) There is less sugar in the acid fractions compared to when a cationic exchange resin is used, which results in less fouling in the acid evaporator and higher sugar recovery.
(3) There is more acid in the sugar containing fractions, which allows for the recovery of the heavy metals and neutralization of pH by addition of lime.
(4) Although the sugar concentration is lower (about 3-9%, more preferably 5-9%) compared to when a cationic exchange resin is used (about 8-14%, more preferably 10-14%), the concentration of the sugar solution can easily be achieved with a reverse osmosis unit or by evaporation. Since concentration of the sugar solution is necessary even when a cationic exchange resin is employed, the additional concentration necessitated by use of an anion exchange or exclusion chromatographic material does not increase significantly the capital or operating cost of the process.
Surprisingly, the aforementioned process allows for the highly efficient and cost effective production of ethanol from cellulose and hemicellulose containing material, e.g. sewage sludge and/or municipal solid waste.